FR2554170A1 - COMBUSTION SYSTEM FOR A GAS TURBINE ENGINE - Google Patents

Abstract

COMBUSTION SYSTEM 9, 99 FOR A GAS TURBINE ENGINE 1 HAS A REACTION LOOP 81, 82, 91, 108, 110, 112 WHICH GENERATES POSITIVE DAMPING FOR THE CONTROL OF COMBUSTION INSTABILITY, ESPECIALLY LOSS IN POST-COMBUSTION SYSTEMS 8 BUT ALSO IN AT LEAST A FEW CASES OF COMBUSTION RUMMING IN MAIN COMBUSTION SYSTEMS 99. THE REACTION LOOP HAS A SENSOR 81, 108, A CONTROL UNIT 82, 110 AND AN EXCITER 91, 112. THE CONTROL UNIT ADJUSTS THE SENSOR SIGNAL STRENGTH, WHICH RECEIVES A PARAMETER OF THE UNSTABLE, CYCILICALLY VARIABLE COMBUSTION PROCESS, AND ADAPTS THE VALUE OF THE PHASE AND AMPLIFICATION OF THE SIGNAL IN ORDER TO OBTAIN A SIGNAL OF THE MINIMUM FORCE SENSOR, ALL OF THIS IS OBTAINED BY TUNING THE EXCITER WITH THE SIGNAL DEPHASE AND AMPLIFYING IN ORDER TO CHANGE THE LIMIT CONDITIONS OF THE COMBUSTION PROCESS AND ELIMINATE BY THE INSTABILITY OF COMBUSTION.

Description

1. The subject of the present invention is a combustion system for an engine.

  at. gas turbines and concerns in particular the control of the phenomenon of unstable combustion as it manifests for example in the post-combustion buzz ("reheat buzz") and

  in the roar of combustion ("Combustion rumble").

  Post-combustion roar is the name given to the noise generated by the oscillations due to combustion in a post-combustion system. He understands

  the vibrations of flame catchers and other elements

  post-combustion system. Oscillations tend to be unstable and can grow in amplitude to threaten integrity

  of the system, either in the short term due to the ex-

  long term exerted on the elements, ie in the long term.

  causes excessive reductions in wear time.

  It is already known to reduce whirring

  post-combustion, either by modifying the combustion chamber

  tion, or by providing for an increase in the thrust of the turbine so as to avoid the combustion conditions 2. known to cause an acute humming problem, having regard to the particular configuration of the system

  considered. However, these palliatives have the consequence of

  this to increase the weight and cost of the post-engine

  bustion or limit the rate of increase in available thrust. In addition, the persistence of a humming problem, even less acute; is not desirable because it also results in long-term wear of the metal,

although at a lower level.

  The roar of combustion is the name given to the noise generated by momentary flow reversals

  in the main combustion chambers of the turbo engine

  gas bine. These momentary flow reversals are not desirable as they can limit the performance of the

  engine. Their origin remains partly unknown. It is possible

  ble than the instability of combustion of the fuel oil mixture

  in the combustion chambers is one of the causes of gron-

  of combustion, but so far we have never

  but attacked the problem only by modifying certain elements.

  The present invention addresses the problem of combustion instability in a gas turbine engine - whether in the main combustion system,

  whether in the afterburner - changing

  managing the boundary conditions of the pro-

  cessation of combustion, in accordance with variations of at least

a chosen parameter.

  According to the present invention, a communication system

  bustion for a gas turbine engine has a feedback loop to control combustion instability

  in the combustion system, the reaction loop

wearing;

  - capture means to capture a parameter

  be of the unstable combustion process, cyclic variant-

  ment, and to transmit a signal corresponding thereto;

  - control means for phase shifting and amplifying

  trust the signal from the capture means; and 3.

  - means of excitation to change the conditions

  limits of the combustion process in accordance with

  ment to the phase shifted and amplified signal; said control means comprising adjustment means for adjusting the signal strength of the sensor and

  vary the value of the phase shift and the amplification

  tion of the signal so as to obtain a signal from the minimum force sensor, means by which the instability of

  combustion is substantially eliminated.

  In more detail, the control means can advantageously comprise: suitable low-pass filtering means, to allow a predetermined range of frequencies of the parameter of the unstable combustion process parameter, passing cyclically; a variable phase shifter for phase shifting the signal leaving the means for picking up a variable quantity; - a variable gain amplifier to amplify the phase shifted signal by a variable amount; and - a monitoring unit to adjust the strength of the signal leaving the low-pass filter and to change the gain of a variable gain amplifier and the phase shift

  produced by the variable phase shifter until the

  min out of the low pass filter is minimal.

  The collection means can include ty-

  various types of transducers capable of supplying the - control means with an electrical signal corresponding to the parameter of the combustion process on which one is acting; for example, can be used vibration, pressure, noise (acoustic pressure) transducers

or optical.

  Likewise, various types of transducers can be used as excitation means, depending on how it was decided to change the boundary conditions of the combustion process. For example, transducers can be used which modulate the flow of a fluid 4. participating in the combustion process. The modulation of the flow can be obtained by causing the variation of the

  pressure in the fluid and in one of the embodiments

  sation of the invention, a reciprocating element such as a diaphragm excited by electromagnetic forces is used to impose variations in

  oil pressure supplying the combustion process.

  In another embodiment, an oscilla-

  speaker-shaped transducer in order to

  produce variations in the sound pressure level

  only in the combustion process and in the vicinity of said process. Embodiments of the invention will now be described by means of examples given with reference to the accompanying drawings, in which: FIG. 1 is a schematic representation in cross section of a gas turbine engine equipped with a system main combustion and a post-combustion system according to the present invention; Figure 2 is a cross-sectional diagram of a conventional Rijke tube apparatus; and Figure 3 is a cross-sectional diagram

  a modified Rijke tube device, equipped with a system

- acoustic control me.

  In Figure 1, there is shown a bypass gas turbine aircraft engine equipped with a main combustion system and a post-combustion system, tion according to the present invention. The aircraft engine 1 comprises a body 3, a bypass duct 5 located

  between the body 3 and the casing 7 which surrounds it and a system

  exhaust post-combustion device 9 comprising a posterior exhaust cone 11 located on the body. of the engine 3, a post-combustion chamber 13 inside the ejection channel. 14, an annular exhaust mixer 15 and a final propulsion nozzle 17. The bypass 5. duct 5 is supplied with air bypassed by a front blower 21, which also supplies the body of the engine 3, the blower 21 being driven by a shaft (not shown in the figure) integral with the final turbine stage 23 of the body 3. The derived air 19 passes through the openings 25 from

  ring mixer 15 at the end of the bypass

  tion and mixes with at least the circumferential part-

  the hot gas 27 afterburner from the turbine

  both from the engine body 3 before the flow resulting from the

mixture does gain the atmosphere.

  The body of the motor 3 comprises a stage of com-

  medium pressure presser 29 which receives low pressure air

  blower 21, a high pressure compressor stage 31, an annular combustion chamber 33, a high pressure turbine stage 35, a medium pressure turbine stage 37 and the low pressure turbine stage 23 mentioned above. The flow of fuel oil in the combustion chambers 33 is controlled by the supply system

fuel oil 99.

  During a large part of the operating time-

  All the thrust from engine 1 is produced by the combustion of fuel oil in combustion chambers 33.

  fuel oil arrives in rooms 33 via supply lines

  100 which are connected at one end to a common supply tube 102 surrounding the

  motor body 3. The supply manifold is supplied

  main line 104 crossing the con-

  branch 5 and protected by a casing, The canali-

  The supply station 104 receives the fuel oil coming from the main fuel oil control unit 106. The installation is subject to the known phenomenon of combustion rumble. In this particular case the phenomenon is caused by the instability of combustion in

  the combustion chambers 33, which produces the momentary inversion

  gas flow in the rooms. The-process 6.

  exact physical, responsible for this instability of com-

  bustion in rooms 33, is the subject of controversy.

  When the aircraft engine 1 is asked for a thrust boost which cannot be obtained by combustion of fuel oil in the combustion stage 33, the post-combustion chamber 13 is then put into service. A control unit 39 post-combustion oil included in the engine oil supply system allows the

  circulation of fuel oil, via the supply lines

  tion 41, in a circular tube 43 supported by

  struts 45 and located concentrically inside

  of the ejection channel 14. The supply lines

  tation are protected by the struts 45. The fuel oil is sprayed into the flame zone by holes (not shown) drilled on the downstream edge of the pipe. A flame catcher or stabilizer 47 comprises a crown with a V-section located downstream of the tube 43

  to generate an area in which tur-

  bulences, to aid combustion, and to create a

  area where the flow speed is reduced, so that the flame

  afterburner is not blown by gases

turbine exhaust 27.

  Post-combustion systems are generally subject to post-combustion roar and it is known that this phenomenon is associated with dynamic instability of combustion in the system. The exact physical process responsible for this instability is controversial.

  It is argued here that in both cases; of combustion-

  tion unstable described above, the process by which unstable combustion occurs is analogous to. the one that appears in a Rijke tube, as we represented on

Figure 2.

  Referring to. Figure 2, we see that a Rijke tube simply consists of a vertical tube 51, 7. often copper or another. metal which is fitted near its lower end by a trellis

  metallic 53 stretched over the entire section of the tube. When-

  that the mesh is heated, whether by means of an electrical resistance (not shown here) or by means of a gas burner 55, an acoustic resonance occurs in the tube. This results from the transfer of heat from the mesh to the air 56 passing through the

  lattice, due to convection in the tube. The combination

  sound of convective updraft with oscillating

  Acoustic reactions to the natural modes of the tube results in greater heat transfer through the air when

  air is condensed only when it is scarce, and if the energy

  Gie gained by the trellis is more important than the energy

  lost at the ends of the tube, or anywhere else,

  the resonant movement accumulates more and more

  gie (i.e., resonance is forced) until the gains and losses of energy balance. The tube mainly resonates at its fundamental frequency like an organ pipe. It should be mentioned that a trellis is not essential for the implementation of this

  experience: a simple flame may suffice.

  More generally, this phenomenon illustrates the

  causes acoustic waves to pass through a source

  that of heat, whether it's a heated mesh or a flam-

  me, can be amplified by the heat source which generates significant acoustic disturbances if the

  heat source is located inside or near

  tee of a resonator. As. the combustion chamber 13 (FIG. 1) emits an intense flame inside the channel

  ejection 14 and near objects having their

  pre resonant frequency, such as the flame catcher

  me 47, it is argued here that the analogy between Rijke's tube and the post-combustion system is fruitful. It is further suggested that an equally useful analogy can be drawn between a Rijke tube and a chamber of 8.

  combustion like that indexed in 33.

  The present invention is based on the consideration

  ration of the factors mentioned above. More generally,

  we propose to control the oscillations of the reso-

  igniters excited by combustion by changing the boundary conditions of the combustion process by means of an active control system using a feedback loop producing a phase shift. This command is given by an appropriate exciter, a vibrating diaphragm for example, which modifies the boundary conditions and changes the nature

  of. fundamental oscillations by increasing the energy per-

  due per cycle of oscillations until resonance does

can establish itself further.

  The essence of the invention is the correction of the instability of combustion, which gives rise, in gas turbine engines, to undesirable effects

  mentioned above E: the absence of active control is

  According to the invention, it can be said that the combustion process

  tion is "negatively amortized" and therefore unstable,

  while active control adds a damping loop

  positive which makes combustion stable. Of course, the invention differs from known "anti-noise" techniques since, according to the invention, the source of the noise is eliminated rather than simply combating its effects. We can

  appreciate this difference by comparing this

  gation to. an article concerning anti-noise techniques given by Professor R. Chaplin of the University of Essex to the English newspaper CME (Chartered Mechanical Engineer)

  from January 1983, pages 41 to 47 which are given to this occ-

case references.

  The method according to the invention has been successfully applied to cancel the instability of combustion in a Rijke tube excited by a flame, as shown in Figure 3. In Figure 3 is shown a tube 51 and a lattice 53 (optional) as described above. However, the gas is supplied by a pipe 61, inside the tube 51, under the trellis 53, so that the gas burns in a flame 63 distributed above the upper surface of the trellis 53. In operation , the tube 51 emits a sound corresponding mainly to its fundamental frequency, the flame 63 visibly manifesting unstable combustion at this frequency. The control system was a closed feedback loop 65. A microphone 67 was enclosed in the upper end of the tube 51 (this end of the tube is very far from the flame 63) to pick up the sound. The signal from the microphone passed through an electronic filter 69 set to let pass only the fundamental mode of the signal (corresponding to the fundamental mode of the tube 51), then passed through a variable phase shifter 71, then was amplified by an amplifier

  variable gain 73, then powered a 75-speaker

  close to the lower end of the tube 51. When

  that the tube resonated due to the instability of the com-

  bustion of the flame 63, it was found that the instability of the combustion, and consequently the resonance, could be reduced or stopped at will by acting on the gain of the amplifier 73 and on the value of the phase shift

  that the phase shifter 71 imposed on the input signal. The observe-

  the flame 63 showed that the visible instability of the flame was eliminated as well as the noise, and it became apparent that once the stability of the flame was obtained

  combustion, the gain of amplifier 73 could be con-

  dramatically reduced. This showed that the power used

  sée to control the oscillations was much less once one had obtained a combustion appreciably

  stab.ie because the noise source had been significantly eliminated

  born; the speaker 75 only had to deliver the power

  this sufficient to maintain stability, that is to say to prevent the establishment of oscillations. In the known 10. anti-noise systems the anti-noise power must be maintained in its entirety in all the operating phases because only the disturbing noise is

fought, not its source.

  We draw attention to the fact that the experience described above highlights both the substantial elimination of the fundamental resonant frequency inside the tube 51 and the substantial elimination of the instability of the '' dynamic heat exchange (manifest in combustion conditions

  unstable flame 63) which gives rise to noise.

  By attacking the fundamental resonant frequency, it is substantially eliminated; at the level of the trellis, the transfer

  preferential heat during the condensation of the colon

  no air resonant and the losses at the limits of the system

  come more important than the input heat, which

helps maintain stability.

  Instead of the microphone 67 used previously, a photomultiplier can be used (not shown

  here) looking at the flame 63 through a half

  black (not shown here) arranged above the upper end of the tube 51. Like the signal recorded by the microphone, the signal recorded by the photomultiplier

  could include a component of the fundamental frequency

  combustion instabilities, and this other para-

  oscillating meter of the process could be used in a

  similar control system to achieve the same results

states than those described above.

  Referring to Figure 1, we can now-

  taking examining the embodiments of the invention.

  First of all, with regard to the poat-combus system-

  tion, it can be seen that during operation of the post-combustion chamber, the noise in the ejection channel 14, including the post-combustion roar, is picked up by a microphone 81 fixed on the external wall of the mixer 11. annular exhaust 15, near the flame catcher 47. The microphone is fixed so that it

  either cooled by the bypass air flow circulating

  the ring mixer. The signal picked up by the micro-

  phone 81 goes into the control unit 82, placed on the outer casing of the engine, where it is filtered by a low-pass filter 83 (see insert) / which only lets through the frequencies of the roaring assumed post-combustion. After being out of phase with the

  variable phase shifter 85, the signal is amplified by an amplifier

  variable gain ficitor 87 then sent to a diaphgrame excited by electromagnetic forces 91, or to a loudspeaker in continuous service which constitutes a part

  from the downstream (truncated) end of the exhaust cone 11.

  Advantageously, the signal is routed to the exciter.

  electromagnetic tor by a cable passing through a strut 12 which is one of the guide bars of

outlet for turbine 23.

  The gain of amplifier 87 and the setting

  of the phase shifter 85 are automatically controlled by a

  monitoring tee 93 which acts on the signal from the filter 83 to determine its strength (amplitude) and which

  modifies the gain of amplifier 87 and the phase shift generated

  dred by the phase shifter 85 to obtain a minimum signal from the filter 83. When the post-combustion roar is detected, the control unit 82 adjusts the vibration of the

  diaphragm 91 to counter the frequency of whirring-

  ment, reducing this. makes the heat entry exciting

  the frequency of the humming and controlling adequately

  te, or even eliminating, the roaring prohlem of a fa-

  lesson analogous to that used for process control

  above taking place in the Rijke tube described above.

  It should be noted that to the extent that the roaring problem at its source has been dealt with, the

  continuous power required by the electromagnetic exciter

  that 92, when the combustion process is stabilized, will be 12.

  much less than that required to bring about stabilization

  station. Variants are possible in the

  control of the post-combustion buzz described above

  above. For example, we can capture various parameters

  of the oscillatory process generated by the

  bustion; the microphone 8-1 could be replaced by

  static or total pressure transducers which

  monitor the variations in pressure associated with the

  burnability around the flame catcher

  me 47. Likewise, the microphone 81 could be replaced by a light guide. resistant to high temperatures, connected to an assembly of photodiodes placed on the external casing of the motor. The assembly of photodiodes would act directly on the frequency of unstable combustion by a variable light emission, as mentioned

  about Rijke's tube. This would have the advantage of

  ter the possibility of an acoustic feedback of the dia-

  phrase 91 on the microphone 81. In another embodiment, a piezoelectric vibration transducer

  could be attached to the outside of the exhaust mixer

  annular 15 to capture the vibrations of the metal

  pickled by the post-combustion roar. In addition, instead of placing the vibrating diaphragm 91 in the exhaust cone, one could consider inserting it into the

wall of the exhaust channel 14.

  Another method of changing the boundary conditions of the unstable combustion process could

  consist in vibrating the flame catcher 47 again

J,

  calf by means of electromagnetic or electric exciters

  This would modulate the flow of fuel oil and the combustion gases around the flame catcher 47, Another possibility is to add

  modulated / bypass air on the compressor to the exhaust flow

  pementde the turbine through a porous surface placed 13.

  in this flow, like a perforated portion of the exhaust cone

  pement 11 arranged like the diaphragm 91, the modulation

  being produced by a valve controlled by the control unit

  mande 82 and working in controlled phase link with the buzzing frequency. This would at all times modulate the relative proportions of air and fuel oil in the combustion process. The modulation of the fuel oil supply pressure in the afterburner system

  in controlled phase link with the frequency of whirring

  This may be a better method for quickly modulating the relative proportions of air and fuel oil. Again, this would correct the instability of combustion at its source (and thereby transfer to the

  buzzing frequency of a cyclic variable energy

only).

  The possibility, mentioned last, of

  modulating the oil supply pressure is specific

  fiquely used to combat the roar of combustion-

  tion of the main combustion system shown in the

  figure 1. The basic technique for producing such a modula-

  tion of the oil flow is naturally usable for

  combat both manifestations of instability

combustion tee.

  In the embodiment of the invention,

  surrounding the main combustion system of the engine, the oscillations of the pressure in the combustion chambers 33 due to the instability of combustion, as was mentioned above are sensed by a vibration sensor 108 fixed inside the coebustion casing 109. The sensor 108 picks up all the vibrations occurring in the casing 109, which include those which are

  due to the phenomenon of roaring combustion; advantageously this cap-

  This may be a piezoelectric transducer resistant to high temperature. The variable voltage generated by the sensor 108 in response to vibrations passes through the unit.

  110 attached to the outer casing 105 of the mo-

  tor. The control unit 110 is similar, in terms of its 14.

  internal operation, at the 82 control unit

  above, unlike filtering the frequencies

  quences, because the frequencies which it lets pass are those of the roar of combustion which has a range of frequencies different from that of the roar of post-

  combustion. The more detailed description of the unit

  82, given above, will therefore suffice for the uni-

* control tee 110.

  The output of the control unit 110 is a si-

  lo gnal capable of acting on a diaphragm (or a piston) exci-

  electromagnetic forces forming part of a

  small chamber filled with fuel oil 112, which is in com-

  munication with main supply line 104. The diaphragm vibrates and thus modulates the pressure

  supplying fuel oil to burners 114 in the rooms

  bres de combustion 33. The modulation is wave-like and

  it is adjusted by the control unit 110 for counter

  the frequency of the roar of combustion, the frequency

  frequency and amplitude of the wave being continuously

  adjusted by control unit 110 to minimize

  mum the signal received by the sensor 108. This has natural-

  the same beneficial effect as that which was

  written about post-com roar control

  bustion by means of the vibrating diaphragm 91 placed in the

  exhaust cone, but the boundary condition of the pro-

  cessation of combustion, which is no longer the same, is the pres-

  Instantaneous fuel oil supply to the flame and not the sound pressure level near the flame. Another means of producing modulation of the

  fuel oil would consist in placing, in the supply line

  fuel oil, a variable closing valve

mandated by a solenoid.

  Although sensor 108 has been described as

  being a piezoelectric transducer detecting vi-

  brations due to the roar of combustion in the combustion chamber, it would be possible to collect the 15 directly.

  corresponding pressure variations by means of a sound-

  of pressure connected to a suitable transducer. The probe would be introduced through the wall of the combustion chambers 33 through one of the dilution air orifices (not shown here) and would be cooled by the air flow circulating there. Similarly, another possibility would be to use an optical probe connected to an assembly

  photodiodes, as mentioned above in pro-

  pos of the embodiment of the post-combustion. With adequate cooling, it would also be possible to place a microphone sensor inside the housing

  109 to capture the roar of combustion-

  tion. The use of a speaker-type exciter has not been mentioned about the system

  for this reason that the installation

  tion of such a device near the

  bustion 33 poses problems which we are cons-

  customers; however, the use of a diaphragm device

  vibrating to counteract the instability of the combustion

  tion by the transmission of pressure waves through the combustion gases in the combustion chamber

  is not to be excluded from the scope of the invention.

  Although a unique means of excitation per system

  was mentioned above, in this case a diaphragm

  vibrating, several similar exciters could be placed in appropriate places in the system, if

it was necessary.

  The control units 82 and 110 can use similar electronic circuits, however for

  that they have good reaction and control speeds

  of, it is preferable that they operate in digital, the control member 93 being a microprocessor and the

  units 83 and 87 with analog converters

  digital and digital-analog as required.

  16. The present invention is not limited to the exemplary embodiments which have just been described, it is on the contrary liable to modifications and

  variants which will appear to those skilled in the art.

17.

Claims (10)

  1 - Combustion system for a turbo engine   gas bine, characterized in that the combustion system (9, 99) comprises a reaction loop (81, 82, 91, 108, 110, 112) for controlling the instability of combustion in the system combustion, the reaction loop comprising; - capture means (81, 108) for capturing a parameter of the unstable combustion process, varying cyclically and emitting a signal corresponding thereto; - control means (82,110) for phase shifting and amplifying the signal emitted by the capture means; and   - means of excitation to change con-   editions at the limits of the combustion process in accordance with   ment to the phase shifted and amplified signal; said control means (82, 110) comprising adjustment means (85, 87, 93) for adjusting the force   of the sensor signal and vary the value of the   wise and amplifying the signal so as to get   a minimum force sensor signal, means through (   which combustion instability is substantially eliminated.   2 - Combustion system according to the claim   tion 1, characterized in that the control means (82, 110) comprise:   - low-pass filtering means (83) suitable for   prayed, to let pass a predetermined range of   frequencies of the combustion process parameter ins-   table, varying cyclically; -. a variable phase shifter (85) for phase shifting the   signal leaving the means (82) for capturing a quantity   variable tee; -an amplifier. variable gain (87) for amplifying the phase shifted signal by a variable amount; and - a monitoring unit (93) for adjusting the strength of the signal leaving the low-pass filter (83) 18. and for changing the gain of the variable gain amplifier (87) and the phase shift produced by the variable phase shifter   (85) until the signal coming out of the filter passes low be minimal.   3 - Combustion system according to claim 1 or 2, characterized in that the capture means (81, 108) comprise vibration transduction means adjusted to detect vibrations due to the process   unstable combustion in the combustion system.   4 - Combustion system according to the claim   tion 1 or 2, characterized in that the collection means   (81, 108) include pressure transduction means   set to detect pressure variations due to the unstable combustion process in the combustion.   - Combustion system according to claim 4, characterized in that the pressure transducing means are a microphone (81) adjusted to detect   variations in the sound pressure level.   6 - Combustion system according to claim 1 or 2, characterized in that the capture means (81,   108) include optoelectro- transduction means   nics set to detect variations in light   emitted from the unstable combustion process.   7 - Combustion system according to one of the res   dications 1 to 6, characterized in that the means of exci-   tation (91, 112) include means for modulating the   flow of a fluid participating in the combustion process.   8 - Combustion system according to one of the res   dications 1 to 6 in which the $ excitation means   (91, 112) include means for generating variations   pressure in a fluid participating in the process combustion.   9 - Combustion system according to one of the res   dications 1 to 8, characterized in that the communication system   bustion (99) has an oil supply (100-106) comprising 19. the excitation means (112), the excitation means comprising a reciprocating element suitable for imposing pressure variations on the supply in fuel oil.   10 - combustion system according to claim 9, characterized in that the reciprocating element   has a diaphragm excited by electromechanical forces   genetic. 11 - Combustion system according to claim   8, characterized in that the excitation means comprise   try a speaker (91, 9.2) to produce varia-   indoor sound pressure level   near the combustion process.   12 - Combustion system according to one of the re-   vendications 1 to 11, characterized in that the system of   combustion is a post-combustion system (9).